A specific burn-up measurement result of a fuel assembly, on which neutrons are irradiated in a nuclear reactor, can be utilized prior to the fuel assembly being loaded on a storage rack of a storage pool for storing and keeping the fuel assembly, a transportation cask or the like for transporting fuel, and the like or when criticality safety and shieldability are checked during fuel dissolution in a reprocessing plant for spent fuel. The specific burn-up measurement result can also be utilized for checking reliability of core management calculation for the nuclear reactor by performing comparison and collation with the core management calculation.
Further, the specific burn-up measurement result can also be utilized for inspection for confirming that the fuel assembly sufficiently burns and an amount of an incorporated nuclear fuel substance decreases. For the purpose of realizing such utilizes, in the past, development of specific burn-up measuring techniques for the fuel assembly has been actively performed in Japan and overseas.
In a fuel assembly for a nuclear reactor, in particular, a light-water reactor of a boiling water type or a pressurized water type, the length of a fuel effective section in which fuel is filled is equal to or larger than 3 m. Specific burn-up indicates a distribution high in the center and low at the upper and lower ends because of a neutron flux distribution in the nuclear reactor. Therefore, to accurately measure specific burn-up of the fuel assembly on which neutrons are irradiated, it is necessary to measure a specific burn-up distribution over the entire length of the fuel assembly. As a specific burn-up measuring technique developed in the past, as disclosed in Patent Documents 1 and 2, there is a method of arranging a large number of detectors for radiation measurement on a side of a fuel assembly and measuring a burn-up profile from signal distributions of the detectors. There is also a method of measuring a burn-up profile over the entire length in an axial direction in a fuel assembly while moving the fuel assembly up and down on a side of a detector (see, for example, Patent Document 3).
Patent Document 1: Japanese Patent Laid-Open No. 1-92692
Patent Document 2: Japanese Patent Laid-Open No. 2-222884
Patent Document 3: Japanese Patent Laid-Open No. 3-273192